What is a thyristor?

A thyristor is really a high-power semiconductor device, also referred to as a silicon-controlled rectifier. Its structure includes four quantities of semiconductor elements, including three PN junctions corresponding for the Anode, Cathode, and control electrode Gate. These three poles would be the critical parts of the thyristor, allowing it to control current and perform high-frequency switching operations. Thyristors can operate under high voltage and high current conditions, and external signals can maintain their working status. Therefore, thyristors are popular in different electronic circuits, including controllable rectification, AC voltage regulation, contactless electronic switches, inverters, and frequency conversion.

The graphical symbol of any Thyristor is usually represented by the text symbol “V” or “VT” (in older standards, the letters “SCR”). Furthermore, derivatives of thyristors also have fast thyristors, bidirectional thyristors, reverse conduction thyristors, and light-weight-controlled thyristors. The working condition of the thyristor is that when a forward voltage is applied, the gate needs to have a trigger current.

Characteristics of thyristor

1. Forward blocking

As shown in Figure a above, when an ahead voltage can be used involving the anode and cathode (the anode is connected to the favorable pole of the power supply, as well as the cathode is connected to the negative pole of the power supply). But no forward voltage is applied for the control pole (i.e., K is disconnected), as well as the indicator light does not illuminate. This implies that the thyristor is not conducting and contains forward blocking capability.

2. Controllable conduction

As shown in Figure b above, when K is closed, along with a forward voltage is applied for the control electrode (known as a trigger, as well as the applied voltage is called trigger voltage), the indicator light turns on. Which means that the transistor can control conduction.

3. Continuous conduction

As shown in Figure c above, after the thyristor is excited, whether or not the voltage around the control electrode is removed (that is, K is excited again), the indicator light still glows. This implies that the thyristor can carry on and conduct. Currently, to be able to cut off the conductive thyristor, the power supply Ea must be cut off or reversed.

4. Reverse blocking

As shown in Figure d above, although a forward voltage is applied for the control electrode, a reverse voltage is applied involving the anode and cathode, as well as the indicator light does not illuminate at the moment. This implies that the thyristor is not conducting and can reverse blocking.

5. In summary

1) When the thyristor is put through a reverse anode voltage, the thyristor is within a reverse blocking state whatever voltage the gate is put through.

2) When the thyristor is put through a forward anode voltage, the thyristor will simply conduct when the gate is put through a forward voltage. Currently, the thyristor is in the forward conduction state, the thyristor characteristic, that is, the controllable characteristic.

3) When the thyristor is excited, as long as there exists a specific forward anode voltage, the thyristor will stay excited whatever the gate voltage. Which is, after the thyristor is excited, the gate will lose its function. The gate only functions as a trigger.

4) When the thyristor is on, as well as the primary circuit voltage (or current) decreases to seal to zero, the thyristor turns off.

5) The disorder for that thyristor to conduct is that a forward voltage needs to be applied involving the anode as well as the cathode, as well as an appropriate forward voltage also need to be applied involving the gate as well as the cathode. To turn off a conducting thyristor, the forward voltage involving the anode and cathode must be cut off, or the voltage must be reversed.

Working principle of thyristor

A thyristor is actually a distinctive triode made up of three PN junctions. It could be equivalently viewed as comprising a PNP transistor (BG2) as well as an NPN transistor (BG1).

1. In case a forward voltage is applied involving the anode and cathode of the thyristor without applying a forward voltage for the control electrode, although both BG1 and BG2 have forward voltage applied, the thyristor remains switched off because BG1 has no base current. In case a forward voltage is applied for the control electrode at the moment, BG1 is triggered to generate a base current Ig. BG1 amplifies this current, along with a ß1Ig current is obtained in the collector. This current is precisely the base current of BG2. After amplification by BG2, a ß1ß2Ig current is going to be brought in the collector of BG2. This current is brought to BG1 for amplification and then brought to BG2 for amplification again. Such repeated amplification forms a vital positive feedback, causing both BG1 and BG2 to get in a saturated conduction state quickly. A large current appears inside the emitters of these two transistors, that is, the anode and cathode of the thyristor (the size of the current is really determined by the size of the burden and the size of Ea), so the thyristor is entirely excited. This conduction process is finished in a really limited time.
2. Following the thyristor is excited, its conductive state is going to be maintained by the positive feedback effect of the tube itself. Whether or not the forward voltage of the control electrode disappears, it is still inside the conductive state. Therefore, the function of the control electrode is just to trigger the thyristor to transform on. Once the thyristor is excited, the control electrode loses its function.
3. The best way to shut off the turned-on thyristor is to decrease the anode current so that it is not enough to keep the positive feedback process. How you can decrease the anode current is to cut off the forward power supply Ea or reverse the connection of Ea. The minimum anode current needed to keep the thyristor inside the conducting state is called the holding current of the thyristor. Therefore, strictly speaking, as long as the anode current is lower than the holding current, the thyristor may be switched off.

What is the difference between a transistor along with a thyristor?

Structure

Transistors usually include a PNP or NPN structure made up of three semiconductor materials.

The thyristor is composed of four PNPN structures of semiconductor materials, including anode, cathode, and control electrode.

Functioning conditions:

The job of any transistor relies upon electrical signals to control its closing and opening, allowing fast switching operations.

The thyristor needs a forward voltage along with a trigger current on the gate to transform on or off.

Application areas

Transistors are popular in amplification, switches, oscillators, along with other elements of electronic circuits.

Thyristors are mostly used in electronic circuits including controlled rectification, AC voltage regulation, contactless electronic switches, inverters, and frequency conversions.

Way of working

The transistor controls the collector current by holding the base current to achieve current amplification.

The thyristor is excited or off by manipulating the trigger voltage of the control electrode to realize the switching function.

Circuit parameters

The circuit parameters of thyristors are based on stability and reliability and in most cases have higher turn-off voltage and larger on-current.

To summarize, although transistors and thyristors can be used in similar applications sometimes, because of the different structures and working principles, they have got noticeable differences in performance and utilize occasions.

Application scope of thyristor

• In power electronic equipment, thyristors can be used in frequency converters, motor controllers, welding machines, power supplies, etc.
• In the lighting field, thyristors can be used in dimmers and light-weight control devices.
• In induction cookers and electric water heaters, thyristors could be used to control the current flow for the heating element.
• In electric vehicles, transistors can be used in motor controllers.

Supplier

PDDN Photoelectron Technology Co., Ltd is an excellent thyristor supplier. It is actually one of the leading enterprises in the Home Accessory & Solar Power System, that is fully active in the progression of power industry, intelligent operation and maintenance control over power plants, solar power and related solar products manufacturing.

It accepts payment via Charge Card, T/T, West Union and Paypal. PDDN will ship the goods to customers overseas through FedEx, DHL, by air, or by sea. If you are looking for high-quality thyristor, please feel free to contact us and send an inquiry.